Boundary-Layer Development at Curved Conduit Entrances

To gain a better understanding of the energy reduction in a conduit inlet shaped according to potential-flow theory, boundary-layer measurements were made in a nonrecirculating air duct with circular, square, and rectangular types of inlet, and the results compared with those obtained from a numerical solution of the boundary-layer equations. The compound elliptical shaped conduit of the U.S. Corps of Engineers was adopted in all three cases. The effect of conduit Reynolds number on the growth of the laminar and turbulent boundary layers in the inlet portion of the conduit was studied. Comparison showed that the energy reduction is higher for a rectangular inlet and least for a circular one. This is because near zero-pressure-gradient conditions are attained sooner in a rectangular inlet as an effect of the elliptic curve of the broad faces. Graphs are given showing coefficients of energy reduction as functions of relative distance into the inlet and Reynolds number.